Aqua Medic DensiTower is a hydrometer cylinder with overflow to determine the exact density by using a hydrometer. Use of Aqua Medic DensiTower: Fill the Aqua Medic DensiTower to the top of the inner tube with the water to be tested, place the hydrometer into the water slowly. Excess water flows into the outer ring. After use, remove the hydrometer and empty the Aqua Meidc DensiTower. Caution: water flows from inner tube and outer ring simultaneously.